The present invention relates generally to optical networking, and more particularly, to a triplexer transceiver that incorporates parallel signal detection for use in passive optical networks (PONs).
The development of optical fiber communication technologies has enabled exponential growth in the capacity of backbone networks. Commercially deployed optical communication systems can now carry ˜3 Tbps in a single fiber, and experimental applications have demonstrated that ultra-dense wavelength division multiplexing (WDM) channels can be transmitted at rates in excess of 10 Tbps. However, current generation access networks, such as digital subscriber line (DSL) and cable hybrid fiber/coaxial (HFC) systems, are constrained by applications such as video-on-demand, video conferencing, large-file transfers, data mirroring, and the like, all of which demand very high bandwidth. The DSL architecture can only support a downstream bandwidth of several Mb/s and an upstream bandwidth of a couple hundred Kbps. Moreover, the transmission distance between any DSL subscriber and a central office is typically limited to 3.4 miles or less. With respect to HFC, traditional cable television systems are not optimized for access network applications. In view of these limitations, optical access networks are ideally suited to building future access networks. The maturity of integration and new packaging technologies, such as un-cooled semiconductor lasers and small form-factor pluggable (SFP) packaging, have enabled optical fiber access networks start to compete with current access network technologies by providing much higher bit rates and better service with reasonable economics.
Fiber optic distribution networks are becoming increasingly important for the provision of high bandwidth data links to commercial and residential locations. Such systems employ optical data transmitters and receivers (“transceivers”) throughout the fiber optic distribution network. These transceivers convert electrical signals to optical signals for optical transmission over optical fibers and receive optical signals from the fibers and convert the modulated light to electrical signals. In active optical networks, the transceivers provide optical-o-electrical-to-optical (OEO) conversion at each node in the network. These elements incorporate high speed electrical circuits in combination with active and passive optical components. Unfortunately, the need to deploy large numbers of transceivers in active optical networks can add considerable costs to the fiber optic network.
The PON architecture eliminates the requirement for OEO conversion, and hence transceivers, at each node of the fiber optic network. In this regard, PONs utilize passive optical components such as beam splitters and filters at network nodes instead of active optical components. A PON therefore has significant cost benefits when compared to active fiber optic networks. PONs have also been designed for two-way, point-to-multipoint data communication, and consequently have significant potential for “last mile” applications where both two-way data transfer and point-to-multipoint broadcast to end users are desired. Accordingly, PONs have many advantages over current access technologies and are expected to be deployed as next-generation access networks. Based on a passive point-to-multipoint network architecture, PONs can support very high transmission bit rates (hundreds of Mb/s or several Gb/s), and numerous broadband services (i.e., Ethernet access, video distribution, voice, etc).
The architecture of a typical PON 100 with a point-to-multipoint architecture is depicted in FIG. 1. An illustrative PON network comprises an optical line terminal (OLT) 102 coupled to a core network(s) 104, a passive optical splitter 106 in communication with the OLT 102, and a plurality of optical network terminals (ONTs)/optical network units (ONUs) 1081, 108m, . . . 108n. The OLT 102 is disposed at the central office and connects the users' local networks 1101, 110m, . . . 110n to the core networks 104. An optical splitter divides the single line into a plurality of equal channels. The ONT provides an interface between the optical network and a user network. This architecture can provide a connection between the OLT and ONT with one fiber using coarse wavelength division multiplexing (CWDM) for bidirectional traffic streams. The downstream traffic from the OLT is broadcasted to all ONTs through the optical splitter, and then each ONT selects traffic addressed to that OLT. For upstream transmission, each ONT can send upstream traffic after getting permission from the OLT. Depending on where the PON terminates, the network can be categorized as fiber-to-the-curb (FTTC), fiber-to-the-building (FTTB), fiber-to-the-premise (FTTP) or fiber-to-the-home (FTTH). By leveraging current commercial optical communication technologies, PON systems can support transmission bit rate of hundreds of Mb/s or several Gb/s, a tenfold increase over existing broadband technologies such as DSL and broadband HFC.
In broadband passive optical networks (B-PONs), the asynchronous transfer mode (ATM) format has been adopted and information can be delivered in accordance with various quality-of-service (QoS) requirements. B-PON upstream transmission rates are 155 Mb/s and 622 Mb/s, and downstream transmission rates are 155 Mb/s, 622 Mb/s and 1.244 Gb/s. In B-PONs, three spectral bands, each having central wavelengths at 1310 nm 1490 nm and 1550 nm, are employed for transmitting upstream data, downstream data and downstream video, respectively. The architecture of a typical B-PON network 200 is depicted in FIG. 2. The B-PON network 200 includes a core network 202 comprising a data network 204 and video network 206. A central office 208 comprises a data OLT 210 and video OLT 212 from which downstream data is communicated at a central wavelength of 1490 nm and to which upstream data is received at a central wavelength of 1310 nm. The video OLT 212 communicates downstream video at 1550 nm. The downstream data and video are combined at 214 and communicated over optical fiber distribution 216 to an optical power splitter 218. The optical power splitter 218 communicates with a plurality of ONTs/ONUs 2201, 220m, . . . 220n to connect the users' local networks 2221, 222m, . . . 222n.
A triplexer transceiver is a key component of a B-PON, and is deployed on the user side or in an optical network terminal (ONT) for transmitting and receiving data and video signals in the three aforementioned wavelength bands. In traditional triplexer transceivers optical downstream data and video signals are separated by optical spectral filters and detected separately. FIG. 3 is a schematic of an illustrative prior art triplexer transceiver 300. In this expedient, the upstream data signal drives a semiconductor laser 302 which operates at a central wavelength of 1310 nm. The downstream video (1550 nm) and data (1490 nm) signals are separated by three-port optical spectral filters 304, 306 and detected separately at photodetectors 308, 310, respectively. The three-port optical filters can be comprised of thin-film type filters. The transmission ports of these filters are configured with a passband to drop the desired channels. Thus, signals outside of this passband are reflected. The optical insertion loss of three-port thin film filters is generally less than 1 dB. For the triplexer structure in FIG. 3, the downstream video signal is dropped by the first three-port filter and experiences minimal loss. The downstream data (1490 nm) and upstream data (1310 nm) signals are separated by the second three-port filter.
In B-PONs, the data and video signals usually have different modulation formats. For the optical upstream and downstream data signals, an electrical data signal modulates the light intensity and an optical baseband signal is generated for transmission. This optical baseband signal can be detected directly. The downstream video signals usually carry tens or hundreds of channels, each channels having a bandwidth of 6 MHz. Subcarrier modulation (SCM) has been adopted for transmission of video signals. With SCM, different video channels are used to modulate radio frequency (RF) carriers at different frequencies. These are then combined and modulate the same optical carrier. For SCM signal detection, a tunable filter selects the different channels, and signal demodulation is accomplished through coherent detection.
In view of the above, it would be advantageous to deploy improved triplexer transceivers in B-PON systems which reduce costs and improve access network performance to provide better quality of service.